Method for reducing formation of electrically resistive layer on ferritic stainless steels

ABSTRACT

A method of reducing the formation of electrically resistive scale on a an article comprising a silicon-containing ferritic stainless subjected to oxidizing conditions in service includes, prior to placing the article in service, subjecting the article to conditions under which silica, which includes silicon derived from the steel, forms on a surface of the steel. Optionally, at least a portion of the silica is removed from the surface to placing the article in service. A ferritic stainless steel alloy having a reduced tendency to form silica on at least a surface thereof also is provided. The steel includes a near-surface region that has been depleted of silicon relative to a remainder of the steel.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. § 119(e) toco-pending U.S. Provisional Patent Application Ser. No. 60/905,219,filed Mar. 6, 2007.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Certain of the research leading to the present invention was funded bythe United States government under Department of Energy CooperativeAgreement DE-FC26-05NT42513. The United States may have certain rightsin the invention.

BACKGROUND OF THE TECHNOLOGY

1. Field of Technology

The present disclosure relates to methods for limiting the formation ofan electrically resistive surface layer or “scale” on stainless steelswhen the steels are subjected to high-temperature, oxidizing conditions.The present disclosure also relates to stainless steels and articles ofmanufacture including stainless steels, wherein the steels have areduced tendency to form electrically resistive scale thereon when thesteels are subjected to high-temperature, oxidizing conditions.

2. Description of the Background of the Technology

Fuel cells are energy conversion devices that generate electricity andheat by electrochemically combining a gaseous fuel and an oxidizing gasvia an ion-conducting electrolyte. Fuel cells convert chemical energydirectly into electrical energy in the absence of combustion, providingsignificantly higher conversion efficiencies than reciprocating engines,gas turbines, and certain other conventional thermomechanical energyproduction devices. In addition, for the same power output, fuel cellsproduce substantially less carbon dioxide emissions than fossilfuel-based power generation technologies. Fuel cells also producenegligible amounts of SO_(x) and NO_(x), the main constituents of acidrain and photochemical smog.

Several types of fuel cells have been developed, differing primarily inthe materials utilized as the fuel cell electrolyte. NASA originallydeveloped alkaline fuel cells including a liquid electrolyte in the1960's to power Apollo and other spacecraft. Liquid electrolytes,however, typically are corrosive and can be difficult to handle. Solidoxide fuel cells (SOFCs), in contrast, are constructed entirely ofsolid-state materials and employ a fast oxygen ion-conducting ceramicmaterial as the electrolyte. SOFCs operate in a temperature range ofabout 500° C.-1000° C. to facilitate solid-state transport. Theadvantages of SOFCs include high energy efficiency and relatively fewproblems with electrolyte management. SOFCs also produce high-gradewaste heat, which can be used in combined heat and power devices, andharnessed for internal reforming of hydrocarbon fuels.

A single SOFC subunit or “cell” includes an anode and a cathode,separated by the electrolyte. During operation of the SOFC cell, anoxidant (such as oxygen or air) is fed into the fuel cell on the cathodeside, where it supplies oxygen ions to the electrolyte by acceptingelectrons from an external circuit through the following half-cellreaction:

½O_(2(g))+2e ⁻→O⁻²

The oxygen atoms pass through the ceramic electrolyte via solid statediffusion to the electrolyte/anode interface. The SOFC can employhydrogen (H₂) and/or carbon monoxide (CO) as a basic fuel.Operationally, pure hydrogen can be used as supplied. If a hydrocarbonfuel such as methane, kerosene, or gasoline is used, it must first bepartially combusted, or “reformed”, to provide hydrogen and carbonmonoxide. This may be accomplished internally within the fuel cell,aided by the high cell operating temperature and by steam injection. Thefuel gas mixture penetrates the anode to the anode/electrolyteinterface, where it reacts with the oxygen ions from the ion-conductingelectrolyte in the following two half-cell reactions:

H_(2(g))+O⁻²→2e ⁻+H₂O_((g))

CO_((g))+O⁻²→2e ⁻+CO_(2(g)).

These reactions release electrons, which re-enter the fuel cell'sexternal circuit. The flow of electrical charge due to oxygen iontransport through the electrolyte from cathode to anode is balancedexactly by the flow of electrical charge through electron conduction inthe external circuit. The cell's driving force is the need to maintainoverall electrical charge balance. The flow of electrons in the externalcircuit provides useful power.

To generate a reasonable voltage, fuel cells are not operated as singleunits, but instead as “stacks” composed of a series arrangement of manyindividual cells, with an “interconnect” joining and conducting currentbetween the anode and cathode of each of the immediately adjacent cells.A common stack design is the flat-plate or “planar” SOFC (PSOFC), whichis shown in a schematic form in FIG. 1. In the PSOFC 10 of FIG. 1, asingle energy conversion cell 12 includes a cathode 20 and an anode 30separated by the electrolyte 40. An interconnect 50 separates the anode30 from the cathode 60 of an immediately adjacent energy conversion cell14 (not fully shown) within the stack. Thus, PSOFC 10 includes arepeating arrangement of cells, substantially identical to cell 12, withan interconnect disposed between each adjacent cell.

The interconnects are critical SOFC components and serve severalfunctions, including separating and containing the reactant gases,providing a low resistance current pathway to electrically connect thecells in series, and providing structural support for the stack. Theinterconnects must be made of a material that can withstand the harsh,high-temperature environment within the cells, must remain suitablyelectrically conductive throughout the fuel cell's service life, andmust have a coefficient of thermal expansion (CTE) that is sufficientlysimilar that of the cells' ceramic components to ensure that the stack'srequisite structural integrity and gas-tightness is maintained. InitialPSOFC designs utilized LaCrO₃ ceramic interconnects. LaCrO₃ ceramic doesnot degrade at the high SOFC operating temperatures and has a CTE thatsubstantially matches the other ceramic components of the fuel cell.LaCrO₃ ceramic, however, is brittle, difficult to fabricate, andexpensive.

To address deficiencies of ceramic electrolytes, interconnects have beenmade from certain metal alloys. Metallic interconnects are desirable forreasons including their relatively low manufacturing cost, highelectrical and thermal conductivities, and ease of fabrication, whichaids in the formation of gas channels and allows for a high degree ofdimensional control. Alloys proposed for interconnect applicationsinclude nickel-base alloys (such as AL 600™ alloy), certain austeniticstainless steels (such as Types 304, 309, 310 and other alloys in the300 Series family), and certain ferritic stainless steels (such as, forexample, E-BRITE® alloy and AL 453™ alloy). Table 1 provides nominalcompositions for several of the foregoing commercially availablenickel-base and stainless steel alloys, all of which are available fromATI Allegheny Ludlum, Pittsburgh, Pa.

TABLE 1 Composition (weight percent) Alloy Ni Cr Fe Al Si Mn Other AL453 ™  0.3 max. 22 bal. 0.6 0.3 0.3  0.06 Ce + La max. E-BRITE ® 0.15max. 26 bal. 0.1 0.2 0.05 1 Mo AL 600 ™ bal. 15.5 8 — 0.2 0.25 — Type304 8 18 bal. — — — —

Certain characteristics of ferritic stainless steels including at leastabout 16 weight percent chromium make them particularly attractive forPSOFC interconnect applications including, for example, low cost,excellent machinability, and CTEs compatible with conventional ceramicelectrodes. Ferritic stainless steels including 16-30 weight percentchromium and less than 0.1 weight percent aluminum are believed to beparticularly suited for interconnect applications. Specific examples offerritic stainless steels considered suitable for PSOFC interconnectapplications include AISI Types 430, 439, 441, and 444 stainless steels,as well as E-BRITE® alloy. The CTEs of the ceramic electrode materiallanthanum strontium manganate and AISI Type 430 ferritic stainlesssteel, for example, are reported to be about 11-13×10⁻⁶ and about9-12×10⁻⁶, respectively.

Ferritic stainless steels, however, commonly include moderate levels ofsilicon, either as an intentional alloying addition or as a residualfrom the steelmaking process. Silicon is commonly present in ferriticstainless steels at levels of about 0.3 to 0.6 weight percent. Siliconis not commonly added to ferritic stainless steels as an intentionalcompositional element, but it may be added during the melting ofstainless steels as a process element. A portion of the silicon added tothe melt, however, unavoidably makes its way into the steel. Therefore,even though silicon is intentionally added in such cases, it may beconsidered a residual impurity in the steel.

Silicon is detrimental to the operational efficiency of ferriticstainless steel interconnects since it tends to migrate to the steelsurface/scale interface and form a thin, generally continuous, highlyelectrically resistive SiO₂ (silica) layer at the interface. Formationof silica at the interface between the steel and the scale formed on thesteel increases the contact electrical resistivity of the interconnectsover time. This makes it increasingly difficult for electrons to passthrough the interface region between the interconnect and theelectrodes, and thereby progressively impairs the ability of theinterconnects to conduct current between the cells. This process, overtime, can significantly reduce the overall efficiency of SOFCs includingferritic stainless steel interconnects. As such, it is one factorconsidered when selecting a suitable interconnect material from amongthe various available ceramic and alloy materials.

Accordingly, it would be advantageous to provide a method foreliminating or reducing the tendency for electrically resistive silicato form on the surface of ferritic stainless steels when the steels aresubjected to oxidizing conditions, such as conditions to which SOFCinterconnects are subjected.

SUMMARY

One aspect of the present disclosure is directed to a method of reducingthe tendency for formation of an electrically resistive silica layer ona silicon-containing ferritic stainless steel article when the articleis subjected to high temperature, oxidizing conditions when in service.The method includes, prior to placing the article in service, subjectingthe article to oxidizing conditions resulting in formation of silica,which includes silicon derived from the steel, on a surface of thesteel. Optionally, at least a portion of the silica is removed prior toplacing the article in service. In certain non-limiting embodiments ofthe method, the conditions under which the silica forms on the steelsurface include heating the article in an oxidizing atmosphere at atemperature greater than 600° C. for a period of time sufficient to formthe silica.

The ferritic stainless steels that may be included in articles processedby methods according to the present disclosure include anysilicon-containing ferritic stainless steel. Non-limiting examples ofsuch ferritic stainless steels include silicon-containing AISI Type 430stainless steel, AISI Type 439 stainless steel, AISI Type 441 stainlesssteel, AISI Type 444 stainless steel, and E-BRITE® alloy. Given thepresent methods' advantages, the methods are considered particularlyuseful as applied to ferritic stainless steels to be used in SOFCinterconnects.

Another aspect of the present disclosure is directed a method of makinga fuel cell interconnect. The method includes treating asilicon-containing ferritic stainless steel by subjecting the steel tooxidizing conditions under which silica including silicon derived fromthe steel forms on a surface of the steel. Optionally, at least aportion of the silica is removed from the surface. The treated steel issubsequently fabricated into the fuel cell interconnect. The methodreduces the tendency for formation of an electrically resistive silicalayer on the ferritic stainless steel interconnect when the interconnectis subjected to high temperature oxidizing conditions in service.

Yet another aspect of the present disclosure is directed to an articleof manufacture comprising a ferritic stainless steel including at leasta near-surface region that has been depleted of silicon relative to aremainder of the ferritic stainless steel. Such a characteristic reducesthe tendency for the formation of an electrically resistive silica layeron a surface of the article when the article is subjected to hightemperature oxidizing conditions. In certain non-limiting embodimentsaccording to the present disclosure, a method according to the presentdisclosure is applied to the article in order to deplete (i.e., reduceor eliminate) silicon in a near-surface region of the steel. Accordingto certain non-limiting embodiments, the article is a mill product (forexample, a sheet, a plate, or a bar) or a fuel cell interconnect.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of certain non-limiting embodiments of themethods, alloys and articles described herein may be better understoodby reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of an embodiment of a PSOFC.

FIG. 2 is a plot showing the relationship between oxygen partialpressure (right-hand Y axis) and water vapor content (left-hand Y axis,measured as dew point) in hydrogen, and including curves plottingstability limits for various oxides at the indicated range of oxygenpartial pressures as a function of temperature.

FIG. 3 is an Auger compositional profile, normalized to measure bulkcomposition, for a Type 430 stainless steel sample panel annealed inhydrogen for approximately 30 minutes at approximately 1010° C.

FIG. 4 is a plot of ASR values (ohm-cm²) obtained at 700° C. and 800° C.testing temperatures for several ferritic stainless steel samplesprepared as described herein.

FIG. 5 is a plot of ASR values (ohm-cm²) obtained at a 800° C. testtemperature in air for various steel-ceramic-steel “sandwich” assembliesmonitored over 500 hours.

FIG. 6 is a plot of weight change (mg/cm²) over time for various treatedand untreated ferritic stainless steel samples heated in a simulatedanode gas.

The reader will appreciate the foregoing details, as well as others,upon considering the following detailed description of certainnon-limiting embodiments of methods, alloys, and articles according tothe present disclosure. The reader also may comprehend certain of suchadditional details upon carrying out or using the methods, alloys, orarticles described herein.

DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

In the present description of non-limiting embodiments and in theclaims, other than in the operating examples or where otherwiseindicated, all numbers expressing quantities or characteristics ofingredients and products, processing conditions, and the like are to beunderstood as being modified in all instances by the term “about”.Accordingly, unless indicated to the contrary, any numerical parametersset forth in the following description and the attached claims areapproximations that may vary depending upon the desired properties oneseeks to obtain in the alloys and articles according to the presentdisclosure. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as set forth herein supersedes anyconflicting material incorporated herein by reference. Any material, orportion thereof, that is said to be incorporated by reference herein,but which conflicts with existing definitions, statements, or otherdisclosure material set forth herein is only incorporated to the extentthat no conflict arises between that incorporated material and theexisting disclosure material.

As discussed above, ferritic stainless steels commonly include moderateamounts of silicon, either as an intentional alloy addition or as aresidual impurity. During service as metallic interconnects, within theharsh, high-temperature oxidizing environment present in the fuel cellstack, even minor amounts of silicon can readily diffuse to thealloy/scale interface and form a thin, generally continuous,electrically resistive SiO₂ (silica) film. This is particularlyproblematic for most ferritic and superferritic stainless steelsincluding relatively high levels of silicon, e.g., greater than 0.15weight percent, but remains an issue for even ultra-low silicon contentferritic stainless steels, often developed specifically for fuel cellinterconnect applications. The tendency for silicon segregation andoxidation is high and has been observed to occur in alloys includingeven very low levels of silicon. This phenomenon can impair the surfaceelectrical conductivity of ferritic stainless steel interconnects andsignificantly decrease fuel cell efficiency over time.

To address this drawback of ferritic stainless steels, a currentapproach is to minimize the concentration of silicon within alloysintended for interconnect applications. For example, CROFER 22APU alloy,available from Krupp-VDM, Germany, is generally produced with a siliconcontent limited to about 0.10 weight percent. The approach of minimizingsilicon content, however, has several drawbacks. Controlling silicon tovery low levels can be technically difficult and also is expensive,generally requiring the use of premium melting techniques such as vacuuminduction melting (VIM), rather than less expensive air melting inconventional electric arc furnaces. Selecting low-silicon content scrapcan substantially increase raw material costs. Even reducing silicon tolow levels, however, may not be effective due to the extremely strongtendency of silicon within ferritic stainless steels to segregate as athin, semi-continuous oxide surface layer.

In order to address the above-described deficiencies of certain ferriticstainless steels, the present disclosure, in part, is directed to amethod for eliminating or reducing the tendency for formation ofelectrically resistive silica on the surface of ferritic stainlesssteels. More particularly, the present disclosure describes uniquemethods for reducing the formation of an electrically resistive silicalayer on the surface of ferritic stainless steel articles when thearticles are subjected to the high-temperature oxidizing conditionstypically found within SOFCs, conditions to which interconnects arecommonly subjected. Such a method involves treating the article toinduce formation of silica on a surface of the steel. Optionally, atleast a portion of the silica is removed from the surface using asuitable silica removal technique. The article may then, optionally, befurther processed to a suitable form, and subsequently placed inservice. The method alters the silicon content of at least a sub-surfaceregion of the steel so as to inhibit formation of silica when thetreated article is subjected to high-temperature oxidizing conditions inservice.

According to one non-limiting embodiment of the methods according to thepresent disclosure, at least a portion of the silicon in asilicon-containing ferritic stainless steel article is segregated to asurface of the article and oxidized on the surface by “pre-oxidizing”the article for a suitable time in a suitable oxidizing atmosphere. Asused herein in connection with embodiments of the methods according tothe present disclosure, “article” refers to either a mill product suchas, for example, a sheet, a plate, or a bar, and also refers to afinished article of manufacture produced by further processing the millproduct to an intermediate or final form. Also, as used herein inconnection with embodiments of the methods according to the presentdisclosure, “oxidizing atmosphere”, “partially oxidizing atmosphere”,“oxidizing conditions”, and like phrases refer to an atmosphere and/orother conditions promoting the formation of oxides on the surface of aferritic stainless steel article subjected to the atmosphere and/orconditions for a suitable period of time.

Embodiments of the methods according to the present disclosure may beapplied to any silicon-containing ferritic stainless steel. Methodsaccording to the present disclosure are considered particularlyadvantageous when applied to ferritic stainless steel includingrelatively high levels of silicon such as, for example, at least 0.15weight percent silicon, but may be applied to any silicon-containingferritic stainless steel. In general, and without intending to limit thescope of the present disclosure, methods according to the presentdisclosure may be applied to ferritic stainless steels comprising, inweight percentages: 15 to 30 chromium; up to 6 molybdenum; up to 2manganese; up to 1 nickel; up to 1 silicon; up to 1 aluminum; up to 0.1carbon; up to 0.1 nitrogen; up to 1 titanium; up to 1 niobium; up to 1zirconium; up to 1 vanadium; iron; and incidental impurities. Specificnon-limiting examples of ferritic stainless steels to which the methodsof the present disclosure may be applied include AISI Types 430, 439,441, and 444, and E-BRITE® alloy (see Table 1 above), alloys that havebeen proposed for use in fuel cell interconnect applications.

Non-limiting examples of suitable oxidizing atmospheres that can be usedin the pre-oxidizing step include an atmosphere at a suitable oxidizingtemperature principally including hydrogen along with a relatively smallconcentration of oxygen. Other non-limiting examples of suitableoxidizing atmospheres include cracked ammonia or synthetic ammonia,argon or another inert gas or mixture of inert gases, and nitrogen, allof which atmospheres also must include a low concentration of oxygensufficient to suitably oxidize silicon segregated to the alloy surface.An atmosphere including a large nitrogen concentration, however, maypromote nitridation at high temperatures and, thus, is not preferred.Preferably, the concentration of oxygen in the oxidizing atmosphere issuch that the atmosphere selectively oxidizes silicon on a surface ofthe article, while not resulting in the formation on the surface of asignificant level of oxides derived from other elements within thestainless steel.

One embodiment of a method according to the present disclosure includesannealing (heating) the ferritic stainless steel article in an oxidizingatmosphere at a temperature similar to, or preferably in excess of, thetemperature range to which it is expected the steel will be subjectedwhile in service. In this way it is possible to significantly depletesilicon within a sub-surface region of the steel and thereby reduce theamount of silica formed on surfaces of the article when the article issubjected to high temperature oxidizing conditions in service. Morepreferably, the annealing treatment is performed at a temperature thatis at least 100° C. greater, and even more preferably at least 200° C.greater, than the temperature to which the article will be subjectedwhen in service. With respect to ferritic stainless steels to be used asSOFC interconnect material, the annealing is preferably conducted at atemperature in the range of at least 600° C. up to about 1100° C., andmore preferably is conducted at a temperature that is considerablyhigher (for example, at least 100° C. or at least 200° C. higher) thanthe conventional 700-800° C. operating temperature that is common forSOFCs.

According to one non-limiting embodiment of a method according to thepresent disclosure, exposing a ferritic stainless steel article to apartially oxidizing hydrogen-containing atmosphere, preferably includingup to about 1×10⁻²⁰ atmosphere of oxygen, at a suitably elevatedtemperature and for a suitable duration results in the formation ofsilica on the article's surfaces. The silicon for formation of theoxides migrates by solid state diffusion from the bulk of the alloy.Preferably, so as to remove substantial silicon from a near-surfaceregion of the article, the silica layer formed on the steel surface hasa thickness of at least 0.5 microns per millimeter thickness of thesteel.

According to certain non-limiting embodiments of methods of the presentdisclosure, all or a portion of the silica formed during the oxidizingtreatment is removed using a suitable silica removal technique prior toplacing the steel in service. Possible silica removal techniques includemechanical, chemical, and thermochemical techniques capable of removingsilica from the surface of a ferritic stainless steel, preferablywithout also removing a significant amount of the steel underlying thesilica. More preferably, the silica removal technique applied to thesteel will not remove any of the steel underlying the silica to beremoved. Non-limiting examples of possible mechanical silica removaltechniques include mechanical abrasion techniques such as, for example,sanding and grinding. Non-limiting examples of possible chemical silicaremoval techniques include immersing the article in, or applying to thearticle surface, a caustic or acidic liquid that dissolves silica.Non-limiting examples of possible thermochemical silica removaltechniques include immersing the article in, or applying to the articlesurface, a caustic or acidic liquid that dissolves silica and that ismaintained at an elevated temperature suitable to enhance the rate ofdissolution of silica. Those of ordinary skill may readily recognizeother suitable techniques for removing all or a portion of silica formedon a surface of the steel.

The step of “pre-oxidizing” the article, in essence, utilizes thedriving force of oxide formation to segregate at least a portion of thesilicon within the steel to a surface of the steel. It is known thatlow-oxygen atmospheres such as, for example, dry hydrogen atmospheres,remain oxidizing to silicon and certain other alloy ingredients thathave solid state mobility and an extremely high affinity for oxygen. Theoxygen content of hydrogen is generally determined by assessing theresidual water vapor content of the gas since oxygen and water arerelated through the well known water shift reaction:

H₂O(g)

H₂(g)+½O₂(g).

FIG. 2 is a plot showing the relationship between oxygen partialpressure (right-hand Y-axis) and water vapor content (left-hand Y-axis,measured as dew point) in hydrogen. As suggested by the above watershift reaction, as the water vapor content of a gas increases, theoxygen partial pressure within the gas also increases. FIG. 2 alsoincludes curves plotting stability limits for various oxides, at theindicated range of oxygen partial pressures, as a function oftemperature.

Given the relationship of the various oxide stability limits at a giventemperature, as shown in FIG. 2, the present inventor concluded that byincluding an oxygen partial pressure that is not too high in an elevatedtemperature atmosphere and also including hydrogen or another suitablenon-oxidizing gas or non-oxidizing gas mixture, silicon can beselectively segregated to the alloy surface and oxidized to form silica,while leaving substantially unaffected and in metallic form within thebulk of the alloy other elements such as, for example, manganese andchromium. As FIG. 2 suggests, oxides of silicon are stable at much loweroxygen partial pressures than oxides of chromium and various otherelements present in stainless steels. The present inventor concludedthat these principles can be applied to ferritic stainless steels toselectively promote silicon migration/segregation and oxidation withoutsignificantly promoting migration/segregation and oxidation of chromiumand various other alloying elements within the steels. The oxygenpartial pressure in the oxidizing atmosphere used in the methodsaccording to the present disclosure preferably is below, and morepreferably is just below, the oxygen partial pressure at which oxides ofchromium are stable and will form on the steel. For example, the oxygenpartial pressure may be up to about 1×10⁻²⁰ atmosphere.

Selective segregation of silicon to the alloy surface depletes the steelof silicon without significantly affecting the concentration of variousother alloying elements within the steel. Using this technique, ferriticstainless steels may effectively be “pre-oxidized” and depleted of allor a significant concentration of silicon, rendering the steels moresuitable for use in applications in which the formation of silica on thesteel surfaces is detrimental. Such applications include, for example,those wherein ferritic stainless steel is used to form interconnects forSOFCs, in which case the silica increases contact resistivity of theinterconnects. Once selectively segregated to the alloy surface andoxidized, the resulting silica can then be removed from the surface, ifdesired. Thus, by selecting suitable pre-oxidizing conditions, certainembodiments of methods according to the present disclosure promoteformation of silica on surfaces of silicon-containing ferritic stainlesssteel and thereby result in significant depletion of the silicon in atleast a near-surface region of the alloy, preventing or reducing thetendency for silica to form on surfaces of the steel when latersubjected to high temperature oxidizing conditions in service.

The following examples describe tests that were conducted and confirmedthe utility and operability of methods according to the presentdisclosure.

EXAMPLE 1

A 0.5 mm thick sample panel of AISI Type 430 stainless steel, whichnominally includes 0.4 weight percent silicon, was annealed in a furnacechamber having a hydrogen atmosphere including a small concentration ofwater vapor, along with incidental impurities. The dew point of thehydrogen atmosphere was not measured but was believed to be in the rangeof about −20° C. to 0° C. The panel was heated in the furnace chamber atapproximately 1010° C. for 30 minutes time-at-temperature (as measuredby a contact thermocouple). The sample panel emerged from the furnaceafter heating with a dull surface tint, indicating that a relativelythick silica-containing layer (scale) had formed on the panel surface.The test panel was then examined using a scanning Auger microprobehaving a depth profiling capability (via an ion sputtering gun). FIG. 3illustrates the Auger compositional depth profile of the stainless steelsample, normalized to measure bulk composition. FIG. 3 plots relativeenrichment of iron, silicon, and chromium in the oxide layer at variousdepths. Significant segregation of silicon from the alloy bulk to thealloy surface was detected, with an approximately 0.18 micron (180 nm)thick silica layer evident on the surface. The scale/alloy interface(i.e., the original surface of the steel), which was at about 0.18micron measured from the surface of the scale, is indicated by thevertical line at about the mid-point of FIG. 3. FIG. 3 also shows thatonly very minor segregation of chromium toward the scale surfaceoccurred as the silica-containing layer formed. FIG. 3 further indicatesthat no evident segregation of iron occurred during oxide formation.Thus, the trial confirmed that selective migration of silicon from thealloy bulk and pre-oxidation of the silicon on the alloy surface is aviable method of selectively depleting at least a portion of siliconwithin the alloy.

Calculations suggest that the concentration of silica developed withinthe scale formed on the 0.5 mm thick Type 430 steel sample would consumeapproximately 40% by weight of the total silicon within the steelsample. The rate of scale formation (i.e., thickness of scale formed perunit time) is generally independent of substrate thickness for bulksamples and, therefore, it is expected that thinner samples wouldundergo a greater degree of silicon depletion if heated for a like timeperiod under the same oxidizing conditions. Also, the silicon depletionaffect of the present method is likely to be magnified near the surface,that is, in the near-surface region, of the substrate due to thepresence of a silicon depletion gradient after the exposure to oxidizingconditions. Because the test was conducted at a temperature (1010° C.)that is well above the normal operating temperature range of SOFCs(approximately 700-800° C.), it is believed that the silicon-depletedalloy layer immediately adjacent and underlying the alloy surface wouldpresent a long diffusion distance for re-supply of silicon to the alloysurface from the bulk of the alloy when the alloy is subjected to hightemperature oxidizing conditions in service. This effect would becomplementary to the overall reduction in alloy silicon content providedby the pre-oxidation treatment, and it is believed it would beparticularly helpful to inhibiting formation of silica on the surface ofinterconnects and other articles formed of ferritic stainless steelswhen such alloys are exposed to high-temperature oxidizing conditions,such as the operating conditions within SOFCs. More generally, it isbelieved that additional advantages may be obtained by subjecting thealloy to pre-oxidizing conditions more strongly promoting formation ofsilicon-containing oxides than would be encountered during the servicelife of final components formed from the alloy. Doing so maysubstantially deplete silicon from a near-surface region of the alloyand better inhibit formation of such oxides when the alloy is subjectedto service conditions.

After formation of the silica layer on the test panel, small couponswere cut from the panel. Several of the coupons were left as-oxidized.Other coupons were subjected to a post-oxidation treatment to remove allor a portion of the oxide scale. The scale removal treatments used were(i) immersion of the coupon in 0.1M hydrofluoric acid for about 2minutes and (ii) immersion of the coupon in 1M sodium hydroxide at about60° C. for about 30 minutes. It is believed that the acid treatmentwould remove bulk alloy along with the scale if the coupon were immersedfor a sufficient time period. Thus, the steel preferably is subjected torelatively aggressive removal treatments, such as hydrofluoric acidsolutions, for a relatively limited duration in order to avoid removalof silicon-depleted near-surface alloy. The particular base treatmentused, on the other hand, should leave unaffected the underlying alloysubstrate and, thus, the exposure time may be relatively liberal.

The as-annealed coupons and the coupons subjected to the acid or basesolutions were then oxidized in air at about 800° C. for about 500hours, resulting in the formation of a relatively thick (1-2 micron)oxide scale on the coupons. The scale thicknesses formed on the sampleswere generally similar to scale thicknesses that would be expected toform on ferritic stainless steel interconnects during their servicelife. The as-oxidized coupons and the coupons that were subjected to theacid or base scale removal treatments were then evaluated for areaspecific resistivity (ASR) at either 700° C. or 800° C. ASR is a measureof contact electrical resistivity, with the goal being to achieve as alow an ASR value as possible in order to optimize electricalconductivity at the testing temperature. FIG. 4 is a plot of ASR values(ohm-cm²) obtained at the 700° C. and 800° C. testing temperatures(spanning the typical SOFC operating temperature range) for theforegoing samples as follows: (1) samples of Type 430 stainless steelthat were not subjected to pre-oxidation treatment (“430 Control-1” inFIG. 4); (2) as-oxidized samples, which were pre-oxidized as discussedabove but were not subjected to further processing to remove theresulting oxide scale (“H2”); (3) samples that were pre-oxidized,immersed in hydrofluoric acid solution to remove all or a portion of thescale, and then oxidized at 800° C., as discussed above (“H2-acid”); and(4) samples that were pre-oxidized, immersed in sodium hydroxidesolution, and then oxidized at 800° C., as discussed above (“H2-NaOH”).FIG. 4 shows that the method including pre-oxidation and oxide scaleremoval steps is relatively effective at limiting contact resistance asevaluated at 700° C., and is even more effective as evaluated at 800° C.ASR reductions achieved by application of embodiments of methodsaccording to the present disclosure ranged the from approximately 50%when evaluated at 700° C., to approximately 75% when evaluated at 800°C. Silicon was actually removed from the alloy by the techniquesapplied, and it is likely that silicon mobility within the alloy isgreater at 800° C. and would cause greater problems in terms ofincreased electrical resistivity if not depleted from at least thenear-surface region before the alloy is placed in service.

FIG. 4 shows that the treated samples (2, 3 and 4) had lower ASR valuesat each test temperature than the untreated samples (1). Of thepre-oxidized samples (2, 3 and 4), the acid-cleaned samples (3)exhibited the worst ASR values at both test temperatures. Withoutintending to be bound by any particular theory of operation, it isbelieved that at least a portion of the beneficial silicon-depletedregion beneath the oxide scale of the sample was removed along with theoxide scale by the acid cleaning treatment applied in the testing,allowing for a relatively short path of migration of silicon from thebulk of the alloy. Thus, it is believed that judicious adjustment ofparameters of the scale removal step, if applied, would help to limitundesirable removal of silicon-depleted alloy regions underlying theoxide scale, thereby better inhibiting formation of silica when thealloy is subjected to oxidizing conditions during the service lifetimeof articles formed of the alloy.

The pre-oxidized samples cleaned of silica using the sodium hydroxidesolution (4) performed in the ASR testing in a manner similar to theas-annealed samples (2). This indicates that the silica-containing scaleformed on the as-annealed samples was disrupted during subsequent hightemperature oxidation of the samples. It was not evident from thetesting whether this phenomenon would be present if a relatively thickersilica-containing scale were formed during the pre-oxidation treatment,prior to removing all or part of the scale, if desired, or prior toplacing the steel in service.

EXAMPLE 2

Several coupons of AISI Type 441 stainless steel having the alloychemistry shown in Table 2 (shown in weight percentages) were prepared.Several of the coupons were heated at 1850° F. (1010° C.) in a mesh-beltfurnace in a hydrogen atmosphere including a small concentration ofwater vapor. The water vapor concentration corresponded to a dew pointof nominally −20° C. The coupons were within the heating zone of thefurnace for approximately 30 minutes. The heat treatment produced asilica scale on the surface of the heated coupons, and these coupons arereferred to in this example as the “pre-oxidized” samples. Other couponsof the same steel were not subjected to the heat treatment and arereferred to in this example as “untreated” samples. It is known to forma homogenous single phase manganese cobaltite spinel (“MC”) coating onthe surfaces of ferritic stainless steel SOFC interconnects to protectthe fuel cells from chromium poisoning and to improve interconnectstability. To better simulate in-service conditions, severalpre-oxidized samples and several untreated samples were coated with anMC coating using a third party process before testing.

TABLE 2 Element Concentration (wt. %) Carbon 0.010 Manganese 0.33Phosphorus 0.024 Sulfur 0.0010 Silicon 0.47 Chromium 17.61 Nickel 0.20Aluminum 0.045 Molybdenum not detected Copper 0.070 Niobium 0.46Tantalum 0.001 Vanadium 0.044 Titanium 0.18 Nitrogen 0.012 Cobalt notdetected Tungsten not detected Tin 0.015 Lead 0.0010 Boron not detected

Uncoated pre-oxidized samples, MC coated pre-oxidized samples, and MCcoated untreated samples were tested by placing test samples of the sametype on either side of a thin block of lanthanum strontium manganate(LSM) ceramic. A thin layer of LSM ink was painted on the contactingfaces to better ensure intimate contact between the samples and the LSMceramic. An electrical current was impressed across the steel-LSM-steel“sandwich” using a power supply, and the resulting voltage establishedbetween the steel samples, across the ceramic, was measured. The voltagewas converted to area specific resistivity (ASR) and reported inmohm-cm², which is a normalized measure of the relative ease ordifficulty of electrical current to move across the sandwich. A lowerASR is desirable as it equates to lower contact electrical resistivitybetween the steel samples and the ceramic. As the ASR of an interconnectincreases, the fuel cell output decreases and, therefore, the energygeneration process becomes less efficient. As ASR continues to increaseover time, the fuel cell eventually may stop generating electriccurrent. Therefore, it is desirable to use materials in fuel cellinterconnects with an ASR that is initially low and increases at a veryslow rate.

The sandwiches were held at a furnace chamber temperature of 800° C. inair and the voltage across each heated sandwich was continuouslymonitored for 500 hours in the manner described above. FIG. 5graphically depicts the results of heating the sandwiches in thehigh-temperature oxidizing test environment. A temporary power loss andthe re-equilibration of the test set-up at 165 hours resulted in a gapand discontinuity in each curve of FIG. 5 at that time. Data lossesduring the periods of approximately 245-315 hours and 405-445 hours alsoproduced gaps in the curves of FIG. 5. Nevertheless, FIG. 5 clearlyshows that sandwiches including the MC coated pre-oxidized samples had asignificantly lower ASR over the test period when heated in theoxidizing test atmosphere. These results clearly confirm that treatingthe ferritic stainless steel samples by a pre-oxidizing processaccording to the present disclosure reduced initial ASR and ASR overtime versus a sandwich including coated untreated test samples. Thissignificant reduction in electrical contact resistivity would result ina significant enhancement in the performance and/or service life ofSOFCs incorporating interconnects composed of the pre-oxidized stainlesssteels. As expected, the uncoated pre-oxidized samples produced an ASRthat was greater than the coated pre-oxidized samples. Given the testresults, it is expected that uncoated samples that were pre-oxidizedusing a process according to the present disclosure would result in ASRvalues, both initially and over time, that are significantly less thanuncoated untreated test samples.

EXAMPLE 3

Coupons of the following ferritic stainless steels used in interconnectapplications were prepared: AISI Type 430 (UNS S43000); Type 439 (UNSS43035); Type 441 (UNS S44100); and E-BRITE® alloy (UNS S44627). Couponsof Types 430, 439, and 441 were pre-oxidized to remove silicon fromsub-surface regions of the coupons using the technique described abovein Example 2 (i.e., 1010° C. for 30 minutes). Other coupons were leftuntreated. The coupons were then heated at 800° C. in simulated anodegas (SAG) for a time in excess of 1000 hours, and the normalized weightchange (mg/cm²) of each sample was determined periodically. The SAGconsisted of 4 vol. % hydrogen, 3 vol. % water vapor, and balance argon,and simulated the fuel side environment within a SOFC. The oxygencontent within the SAG was low, but sufficient to oxidize the samples.

FIG. 6 is a plot of the test results. Pre-oxidation (i.e.,desiliconization) was uniformly beneficial to the samples heated in theSAG. The results confirm that the pre-oxidation treatment according tothe present disclosure for removing silicon from subsurface regions offerritic stainless steels reduces the rate of oxidation of thepre-oxidized steels when subjected to environments simulating those towhich an interconnect is subjected within a SOFC.

FIG. 6 shows that E-BRITE® alloy in the untreated state exhibited thelowest weight change of any of the test samples. E-BRITE® alloy,however, is a more costly material given that, for example, it includesat least about 10 weight percent more chromium than the other ferriticstainless steels tested. In any case, it is expected that pre-oxidizingE-BRITE® alloy samples using the technique applied to the other sampleswould have resulted in a reduced weight gain relative to the untreatedE-BRITE® alloy samples.

Accordingly, embodiments of methods according to the present disclosureinvolve subjecting an article (such as, for example, a mill product, aninterconnect, or another part) composed of a silicon-containing ferriticstainless steel to a pre-oxidation treatment adapted to promoteformation of an external surface oxide layer including silica derivedfrom silicon present in the steel. All or a portion of thesilica-containing oxide scale may be removed by a suitable silicaremoval technique such as, for example, a suitable mechanical, chemical,or thermochemical technique. Non-limiting examples of chemical scaleremoval techniques, discussed above, include applying an acid or causticliquid to the scale. It may be advantageous to heat the liquid in orderto speed dissolution of the scale within the liquid and, thus, athermochemical technique (involving a heated chemical) may be preferableto a immersion in a room-temperature liquid bath.

The pre-oxidizing treatment serves to deplete silicon from at least aportion of the substrate, primarily near the steel surface, which inturn decreases the tendency for silica formation on the surface of thesubstrate when subjected to subsequent elevated temperature or otheroxidizing conditions. Removing all or a portion of the silica scaleappears to be beneficial in terms of better inhibiting formation ofsilica when the treated surface is later subjected to oxidizingconditions in service. Nevertheless, methods according to the presentdisclosure also appear to inhibit in-service silica formation even ifthe silica formed on the steel during pre-oxidation step is not removed.In cases in which the silica scale formed during pre-oxidation is notremoved, it is possible that the oxide scale that grows outward when thesteel part is in service disrupts the thin silica scale formed duringpre-oxidation. Absent pre-oxidizing the alloy, however, asemi-continuous silica layer may readily form in situ under the scalethat grows on the alloy when in service, and in that interface regionthe silica disrupts the surface electrical conductivity of the alloy.

The foregoing examples of possible methods, alloys, and articlesaccording to the present disclosure are offered by way of example only,and are not exhaustive of all methods, alloys, and articles within thescope of the present disclosure. Those having ordinary skill, uponreading the present disclosure, may readily identify additional methods,alloys, and articles. Also, those having ordinary skill in the art willbe capable of fabricating the various articles described herein from thealloys described herein and according to the present disclosure, as suchknowledge exists within the art. For example, those having ordinaryskill may readily fabricate fuel cell interconnects from suitablydimensioned ferritic stainless steel mill products. As such, a detaileddescription of the fabrication is unnecessary herein.

Although the foregoing description has necessarily presented only alimited number of embodiments, those of ordinary skill in the relevantart will appreciate that various changes in the details of the examplesthat have been described and/or illustrated herein may be made by thoseskilled in the art, and all such modifications will remain within theprinciple and scope of the present disclosure as expressed herein and inthe appended claims. It will also be appreciated by those skilled in theart that changes could be made to the embodiments above withoutdeparting from the broad inventive concept thereof. It is understood,therefore, that this invention is not limited to the particularembodiments disclosed herein, but it is intended to cover modificationsthat are within the principle and scope of the invention, as defined bythe claims.

1. A method of reducing the tendency for formation of an electricallyresistive silica layer on a silicon-containing ferritic stainless steelarticle subjected to high temperature conditions when in use, the methodcomprising: prior to placing the article in use, subjecting the articleto conditions under which silica including silicon derived from thesteel forms on a surface of the steel.
 2. The method of claim 1, furthercomprising removing at least a portion of the silica from the surfaceprior to placing the article in use.
 3. The method of any of claims 1and 2, wherein subjecting the article to conditions under which a silicaincluding silicon derived from the steel forms on a surface of the steelcomprises: heating the article in an oxidizing atmosphere for a periodof time sufficient to form silica including silicon derived from thesteel on a surface of the steel.
 4. The method of claim 3, wherein themethod depletes a portion of the silicon from a near-surface region ofthe ferritic stainless steel.
 5. The method of claim 4, wherein theferritic stainless steel comprises at least 0.15 weight percent silicon.6. The method of claim 4, wherein the ferritic stainless steelcomprises, in weight percentage: 15 to 30 chromium; 0 to 6 molybdenum;up to 2 manganese; up to 1 nickel; up to 1 silicon; up to 1 aluminum; upto 0.1 carbon; up to 0.1 nitrogen; up to 1 titanium; up to 1 niobium; upto 1 zirconium; up to 1 vanadium; iron; and incidental impurities. 7.The method of claim 4, wherein the ferritic stainless steel is selectedfrom the group comprising AISI Type 430 stainless steel, AISI Type 439stainless steel, AISI Type 441 stainless steel, AISI Type 444 stainlesssteel, and E-BRITE® alloy.
 8. The method of any of claim 3, whereinheating the article comprises heating the article at a temperaturegreater than the temperature to which the article will be subjected inservice.
 9. The method of claim 3, wherein heating the article comprisesheating the article at a temperature at least 100° C. greater than thetemperature to which the article will be subjected in service.
 10. Themethod of claim 3, wherein heating the article comprises heating thearticle at a temperature at least 200° C. greater than the temperatureto which the article will be subjected in service.
 11. The method ofclaim 3, wherein heating the article comprises heating the article at atemperature of at least 600° C.
 12. The method of claim 2, whereinheating the article comprises heating the article at a temperature inthe range of 600° C. to 1100° C.
 13. The method of claim 3, wherein theoxidizing atmosphere is a gaseous atmosphere comprising oxygen at apartial pressure not greater than 1×10⁻²⁰ atmosphere.
 14. The method ofclaim 13, wherein heating the ferritic stainless steel article comprisesheating the article at a temperature in the range of 600° C. to 1100° C.15. The method of claim 3, wherein the oxidizing atmosphere is a gaseousatmosphere consisting essentially of hydrogen, a partial pressure of nomore than 1×10⁻²⁰ atmosphere oxygen, and incidental impurities.
 16. Themethod of claim 15, wherein heating the article comprises heating thearticle at a temperature in the range of 600° C. to 1100° C.
 17. Themethod of claim 3, wherein heating the article in an oxidizingatmosphere comprises heating the article at a temperature of at least600° C. for at least 2 minutes time-at-temperature.
 18. The method ofclaim 3, wherein the silica formed on the surface is a layer having athickness of at least 0.5 microns per millimeter thickness of thearticle.
 19. The method of claim 2, wherein removing at least a portionof the silica comprises subjecting the silica to at least one of amechanical, chemical, and thermochemical treatment.
 20. The method ofclaim 2, wherein removing at least a portion of the silica comprisescontacting the silica with a liquid comprising a compound selected fromthe group consisting of sodium hydroxide, hydrofluoric acid, nitricacid, and hydrochloric acid for a period of time sufficient to removethe portion.
 21. The method of claim 19, wherein removing at least aportion of the silica does not remove steel underlying the silica fromthe article.
 22. The method of any of claims 1 and 2, wherein thearticle is selected from the group consisting of a mill product, asheet, and a fuel cell interconnect.
 23. An article of manufacturecomprising a ferritic stainless steel including a near-surface regiondepleted of silicon relative to a remainder of the ferritic stainlesssteel.
 24. The article of manufacture of claim 23, wherein the articlehas a reduced tendency to form an electrically resistive silica layerincluding silicon derived from the steel when the article is subjectedto high temperature oxidizing conditions.
 25. The article of manufactureof claim 23, wherein the ferritic stainless steel is selected from thegroup comprising AISI Type 430 stainless steel, AISI Type 439 stainlesssteel, AISI Type 441 stainless steel, AISI Type 444 stainless steel, andE-BRITE® alloy.
 26. The article of any of claims 24 and 25, wherein thearticle is a fuel cell interconnect.
 27. A method of making a fuel cellinterconnect, the method comprising: treating a silicon-containingferritic stainless steel by subjecting the steel to conditions underwhich silica including silicon derived from the steel forms on a surfaceof the steel; and fabricating a fuel cell interconnect from the treatedsteel.
 28. The method of claim 27, wherein treating thesilicon-containing ferritic stainless steel further comprises removingat least a portion of the silica from the surface prior to fabricatingthe fuel cell interconnect.
 29. A method of treating a fuel cellinterconnect comprising a silicon-containing ferritic stainless steel toreduce the tendency for formation of an electrically resistive silicascale on a surface of the interconnect when subjected to hightemperature conditions in service, the method comprising: prior toplacing the interconnect in service, subjecting the interconnect toconditions under which silica including silicon derived from thestainless steel forms on a surface of the stainless steel.
 30. Themethod of claim 29, wherein the method further comprises prior toplacing the interconnect in service, removing at least a portion of thesilica from the surface of the stainless steel.